For cake:
Preheat oven to 350°F. Butter 10-inch-diameter springform pan. Line bottom with waxed paper. Stir butter and chocolate in heavy large saucepan over low heat until melted. Mix sugar and cocoa in large bowl. Add eggs; whisk until well blended. Whisk in chocolate-butter mixture. Pour batter into prepared pan. Bake until tester inserted into center comes out clean, about 45 minutes. Cool cake completely in pan on rack. Run knife around pan sides to loosen cake. Release pan sides. (Can be made 1 day ahead. Cover and refrigerate.)

For caramel sauce:
Stir sugar, water, and lemon juice in heavy medium saucepan over low heat until sugar dissolves. Increase heat; boil without stirring until syrup is deep amber color, about 7 minutes. Remove from heat. Add in cream (mixture will bubble vigorously). Return to low heat; stir until any bits of caramel dissolve. Add butter; whisk until smooth. (Can be made 1 day ahead. Cover and chill.)